plasma protein binding
When a drug enters the blood, much of it does not float free — it clings reversibly to proteins dissolved in the plasma, chiefly albumin and, for some drugs, a glycoprotein. Picture the drug molecules as passengers and the proteins as buses: a molecule riding the bus is bound and inactive, while only those who have stepped off — the free fraction — can leave the blood, reach the target, and be eliminated.
The binding is loose and constantly reversing, so bound and free drug stay in equilibrium: as free drug is consumed, bound drug releases to replenish it. This means proteins act as a reservoir, smoothing out and prolonging a drug's presence. Only the free fraction is pharmacologically active and available for distribution, metabolism and excretion.
Plasma protein binding shapes a drug's distribution, its measured concentration, and its apparent potency. Highly bound drugs tend to stay in the blood with a small volume of distribution. While dramatic drug interactions from one drug displacing another off albumin are often overstated, protein binding remains essential for interpreting blood levels, since most assays measure total drug while only the free portion acts.
Warfarin is about 99 percent bound to albumin, so its small free fraction does the work; because only the free drug acts, medicinal chemists track free rather than total concentration when judging in-vivo potency.
Free, not total, concentration drives the effect.
It is the free, unbound concentration that drives effect — not the total. A drug that is 99 percent bound is not weak; only its free 1 percent need reach the target, and binding simply buffers its supply.